Esterification of Glycerol with Acetic Acid over Highly Active and Stable Alumina-based Catalysts: A Reaction Kinetics Study

Recently, biomass-derived energy has been receiving more and more attention due to the depletion of conventional fossil sources at a faster rate, as well as major environmental issues.1 Biodiesel obtained by transesterification of vegetable oils and animal fats provides an alternative fuel option for the future.2 Transesterification of oils and animal fats produce glycerol in large excess (10 kg glycerol/90 kg of biodiesel) as a byproduct.3 In recent decades, with the rapid development of the biodiesel industry, glycerol has been oversupplied in the market, resulting in a significant decrease in the price of glycerol. Therefore, it is essential to develop a new glycerol value-added process to make the overall biodiesel process cost-competitive and environmentally friendly.4,5 Various glycerol value-added processes such as esterification, hydrogenolysis, etherification, carboxylation, fermentation, production of hydrogen and syn-gas have been reported in the literature.5–7 Among all the processes explored, esterification of glycerol with acetic acid can be a good choice of glycerol utilization. The primary products of glycerol esterification i.e., MAG, DAG and TAG has great industrial significance. MAG is used as an additive in the food industry and in the manufacture of explosives. DAG and TAG are used to manufacture inks, softening agents, and plasticizers.3,8,9 DAG and TAG are also useful fuel additives for reducing viscosity and improving anti-knocking property of gasoline.10,11 Moreover, MAG, DAG and TAG are also used in cryogenics as well as in the biodegradable polymer industry.12–14 Traditionally, mineral acids such as H2SO4, HCl, or H3PO4 are used as homogeneous catalysts for the glycerol esterification reaction.15–17 However, such processes are accompanied by drawbacks such as catalyst separation, product purity, necessity of neutralization, and reactor corrosion.18 There are several studies reported in the literature regarding the use of different heterogeneous catalysts for the esterification of glycerol with acetic acid.3,9,11–13,19–22 Zeolites have been reported to give poor selectivity to DAG and TAG, mostly due to their small pore size.3 Several studies have reported that the acidity of the catalyst is an important factor affecting conversion and product selectivity.9,11 Some studies have discussed the use of heteropolyacids immobilized on different supports like silica8, zirconia11, activated carbon12, and zeolites19. However, low selectivity to TAG has been reported, and some loss in activity on reuse due to leaching has been observed. SnCl2 was studied as a less corrosive alternative; however, the selectivity to TAG was poor.20 A two-step reaction using acetic anhydride in the Esterification of Glycerol with Acetic Acid over Highly Active and Stable Alumina-based Catalysts: A Reaction Kinetics Study